The shape memory effect or shape memory functionality is a phenomenon already known and established for metal alloys as well as polymers. Shape memory functionality is the ability of a material to temporarily fix a second shape after an elastic deformation and only recover the original (first) shape after application of an external stimulus. The advantageous and intriguing properties of shape memory materials are in particular the possibility to initiate a desired change in shape by an appropriate external stimulus, so that an original shape, after deformation, is re-established, and the possibility to deform and program these materials so that highly specific configurations and shape changes can be obtained. The deformed shape is often called the temporary shape in the art. The phenomenon is a functionality and not an inherent material property. The effect/functionality results from a combination of material structure and specific functionalization processes.
As already indicated above, the first materials known to provide this functionality were metallic shape memory alloys. In the recent past, shape memory polymers have been developed. Typical shape memory polymers are, for example, phase segregated linear block copolymers, having a hard segment and a switching (soft) segment, typically thermoplastic materials. Other examples are systems comprising soft segments (switching segments) and chemical, e.g. covalent network points/crosslinks, which provide the function of the hard segments, typically in network materials. Important representatives of these types of materials are disclosed in the international publications WO 99/42147 and WO 99/42528. These materials employ as external stimulus for initiating the recovery of the original shape a change in temperature, usually a temperature rise. Shape memory polymers being susceptible to other external stimuli are also known, such as the photosensitive shape memory polymers disclosed in WO 2004/062706.
The shape memory effect or function, as seen with shape memory polymer as identified above, may be explained on the molecular level as follows.
The hard segments or the above described chemical network points are responsible for the original (first) shape by providing stable crosslinks, which may be chemical (covalent) or physical (ionic, coordinative, other physical interactions). These crosslinks provide strong interactions within the material, so that these crosslinks establish the permanent (i.e. original) shape. The switching (soft) segments likewise provide crosslinks, at least to a certain extent, crosslinks which may, however, be loosened or destroyed upon application of a suitable external stimulus. These crosslinks again may be of physical or chemical (e.g. covalent) nature. Soft segments in this respect define segments which are responsible for fixing the temporary shape, i.e. switching segments. A shape memory polymer, in the form as established by the hard segments or the chemical network points, may be deformed by appropriate processes, for example, by heating to a temperature above the transition temperature (for temperature-dependent shape memory polymers) of the switching segments. In the deformed state, the shape memory polymer is then fixed and cooled, so that crosslinks are established between the switching (soft) segments, holding the shape memory polymer in the deformed, i.e. temporary, shape. After application of a suitable external stimulus, in this example a new rise in temperature to above the transition temperature of the switching segments, the crosslinks within the switching segments fixing the temperature shape are loosened (destroyed), so that the material recovers the original, i.e. permanent, shape defined by the hard segments.
Various shape memory polymers which provide the above described shape memory functionality have been developed in recent years. However, such shape memory polymers are not applicable in all technical fields, either due to mechanical restrictions or cost considerations. Accordingly, there exists the need to develop novel materials which enable the provision of shape memory functionality which either show better mechanical properties, such as hardness, stiffness etc., compared with conventional shape memory polymer and/or which can be prepared at lower costs, compared with the conventional shape memory polymers.